This invention relates to a journal and bearing therefore, and more particularly, to assemblies for use in automotive suspension systems.
In an automotive front suspension system, the front wheels are secured to a corresponding front axle which is secured to a spindle. The spindle is connected to an upper and a lower arm by respective ball joints. The upper and lower arms, in turn, are connected to the automotive chassis by upper and lower control arm bushings. These bushings utilize elastomeric material and have a significant effect on ride, comfort, handling characteristics, road noise and vibration isolation. These bushings are torsional spring type devices with limited oscillatory capability and parasitic torsional spring rate. The bushings comprise three components: A rubber element compressed between two metal sleeves. The rubber element is secured by high compression or is chemically bonded to one or both sleeves, which are steel tubes. The rubber element is used to perform several functions simultaneously, i.e., provide vibration damping or isolation and rotary motion between the inner and outer sleeves, and thus poses conflicting material development and design problems. As a result there is a need for cost effective, low torsional rate tunable isolation pivot joints to replace currently used torsional spring type rubber bushings.
To fill this need, numerous devices have appeared commercially, applying new principles, but none have met the requirements for performance, economy and low torsional rate tunable isolation suspension in pivot joints such as in the control arm bushings mentioned above. Low torsional rate is required between the inner and outer sleeves to reduce parasitic loads induced by a solid elastomeric element when one portion is rotated relative to another portion as occurs in suspension applications.
Some prior art designs include Teflon cloth lined rubber bushings and a silicone grease lubricated rubber bushing. Although both devices are based on a sliding bearing principle, none offer a relatively low torque friction and load carrying capacity needed. For example, FIG. 5 illustrates various bushings at static no load conditions. Curve a illustrates a torsional spring oscillatory bushing comprising a rubber block between inner and outer tubes locked together by high compression as manufactured by the assignee of the present invention. This bushing exhibits 13 in.-lb./degree of parasitic torque response.
Curve b illustrates the characteristics of a dual torsional spring oscillatory bushing referred to as a duplex bushing using several solid rubber elements and steel tubes and exhibits a parasitic torque response of 8 in.-lb./degree. Curve c illustrates a Teflon cloth lined rubber bushing providing a sliding bearing. This bushing exhibits 120 in.-lb. of running torque. Curve d illustrates the characteristics of another product of the assignee of the present invention comprising silicone lubricated rubber sliding on a steel or plastic journal. This bushing exhibits 70 in.-lb. of running torque. All of the above exhibit too high a running torque response and objectionable break-away torque for certain automotive suspension system applications, such as in the control arm bushings described above.
Generally, bushings currently available are rubber embedded, grease lubricated, or plastic on metal wherein the plastic is premolded to the desired dimensions and assembled in mating metal components such as used in certain ball joints, or still other arrangements. All require high surface finish and close tolerance corrosion protection, and are relatively costly. The present inventor recognizes a need for a low cost, low torsional rate tunable bushing, e.g., responsive to torques below 25 in.-lb., and which can carry relatively high loads and is durable.
A method of making a bearing structure according to the present invention comprises forming a first member with a first surface of a first radial dimension relative to a first axis; forming a second member having a second surface with a second radial dimension smaller than the first radial dimension relative to a second axis; positioning the first member relative to the second member with the first and second surfaces forming an interface region in facing spaced relation and the axes coaxial; and molding an intermediate thermoplastic journal member between the first and second surfaces and which rotates relative to at least one of the first and second members.
The method according to one embodiment comprises shrink fitting the intermediate member to the second member to preclude relative rotation therebetween in response to a torque applied between the first and second members and shrinking the intermediate member relative to the first member first dimension to form the intermediate and first members into a respective journal and bearing.
In accordance with a further embodiment, the members are tubular.
In a further embodiment, the method includes forming at least one projection on one of the first and second members and at least one mating recess engaged with the projection on the intermediate member at the interface therebetween to form an axial thrust absorbing arrangement.